The present invention relates to a cold cathode tube which produces light intensity or luminous intensity uniform with respect to a longitudinal direction of the tube for use in back illumination of a transmitting-type liquid-crystal display apparatus, back illumination of an ornamental or decorative sign board or billboard, or the like.
A cold cathode tube of the kind referred to above has conventionally been arranged as shown in FIG. 2 of the attached drawings. Specifically, a cold cathode tube 1 is arranged such that a pair of electrodes 3 and 4 are disposed respectively adjacent to both ends of a tube element 2 which is composed of a cylindrical glass material or the like.
According to the cold cathode tube 1 arranged in this manner, voltage from an electric power source section 5 is applied to the electrodes 3 and 4, whereby electric discharge occurs between tile electrodes 3 and 4. Electrons so discharged are impinge against a fluorescent or luminescent material coated on an inner wall of the tube element 2, whereby a light is emitted. At the start of the electric discharge, the voltage applied to the electrodes 3 and 4 is set relatively high. After the initial electric discharge, the voltage is reduced to simply maintain the electric discharge. Thus, an attempt is made to reduce electric-power consumption and to miniaturize the power-supply transformer to reduce cost.
In the cold cathode tube 1 having the arrangement, as shown in FIG. 2, a deactivated condition may be generated due to a voltage drop after start of the electric discharge, with respect to the electrode to which a longer one of lead wires to the respective electrodes 3 and 4 from the power source section 5 is connected, that is, in the illustrated example, tile electrode 4. Thus, luminous intensity is not uniform over an entire length of the tube element 2.
In FIG. 3, a cold cathode tube 1' is known in which the cold cathode tube 1' is provided with an auxiliary electrode 6 formed of an electric conductive material so as to extend over the entire length of the tube element 2 along its longitudinal direction at an outer surface of tile tube element 2. A deactivated condition is eliminated near the electrode 4 to which the longer lead wire is connected.
According to the cold cathode tube 1' arranged as described above, electric discharge is conducted by provision of the auxiliary electrode 6 with respect to the electrode 4 to which the longer lead wire is connected, whereby there exists a sufficient amount of electrons. Accordingly, a deactivated condition does not occur near electrode 4.
In the cold cathode tube 1' arranged in this manner, however, when voltage after start of the electric discharge similarly drops at an area in the vicinity of the center of the tube element 2, a deactivated condition may occur. This is because electrons generated due to the electric discharge are absorbed by the auxiliary electrode 6 which extends over the entire length of the tube element 2.